Answer carefully, especially since this Minneapolis Star Tribune article cautions that when it comes to exes, Facebook friends are rarely "just" friends. An excerpt:
So if you're tempted to reconnect because reconnection is instantaneous and offers what seems like a harmless thrill, think again about your motives: You probably have all the friends, on Facebook and otherwise, that you need.
And, assuming your marriage is solvent and dear to you, either ignore a new "friend" notification or accept the face from the past and post a benign, polite message, says Jack Quesnell, relationship therapist for the Midwest Center for Personal and Family Development in Minneapolis.
Be honest: How many times have you accepted an ex's friend request solely for the sake of friendship?
Yeah, that's what I thought.
(via YourTango)